The present invention relates to a process for reducing or avoiding foam production during chemical and physical materials conversion processes in a liquid medium, wherein the materials conversion is performed in a hopper-shaped reactor with thorough mixing of the contents of the reactor. The invention also relates to a device for performing the process.
In many chemical and physical materials conversion processes more or less intensive foam production occurs which makes process control difficult or even impossible. Thus specific measures are required in order to break down the foam and/or to avoid the production of foam or at least to reduce it to an acceptable level. Although foam production can sometimes be avoided or reduced by managing the flow conditions in the reactors, for instance by avoiding sharp bends in the flow and locating liquid inlets below the surface of the liquid, in many cases these types of action are not enough. Accordingly, processes for breaking down the foam, including thermal, chemical and mechanical processes, have to be used. A review of this topic is provided by Pahl et al., Chem.-Ing.-Tech. 67 (1995), 300-312. The process engineering involved becomes more complicated when using known measures for breaking down foams and/or the product purity of the product being produced is reduced by the use of chemical antifoam agents. In addition, the production costs are increased.
A variety of reactors have been disclosed for performing batchwise and continuous crystallization processes in which foam problems often occur due to intensive internal or external circulation of the liquid reaction medium containing the growing crystals. A review of this topic is provided in Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. (1988) vol. B2, 3-22 to 3-25. For example, a crystallizer with forced external circulation includes an evaporation tank, a circulation system with pumps and heat exchanger as well as a feed pipe for the solution being supplied and a withdrawal port for the crystal suspension. Over and above the problems associated with foam production, secondary seed crystal formation occurs as a result of the mechanical circulation system and this leads to the production of a finely divided product, while there is a risk of incrustations forming in the heat exchanger.
Vacuum crystallizers with internal circulation of the crystal suspension, for example a Swenson or Standard-Messo crystallizer, contain a guide tube located in the tank or a variety of shapes of guide metal sheeting and an agitator. It has been shown that, even in these types of crystallizers, foam is often produced which then requires the use of antifoam agents or special devices for breaking down the foam. Here again, the particles are broken down due to the input of mechanical energy via the agitator and as a result of deflections on encountering the baffles. Although the proportion of fine particles is reduced by reducing the speed of rotation of the agitator, incrustations and problems due to insufficient mixing then occur. These types of problems have been observed in practice when preparing sodium perborate in accordance with the process described in CAV 1973, pages 45-50.
According to classical crystallization theory, although poor particle number control in crystallizers of the type mentioned above can be improved by chemically influencing the production of seed crystals or by the dissolution of seed crystals or by structural separation of the seed crystal production and particle growth zones, these types of measures are associated with additional expense. Although the process according to EP-B 0 452 164 makes use of these types of measures during the preparation of sodium perborate, the foam problem is not solved and, depending on the choice of surface-active substance used in this process, may even be intensified.
The hydrodynamic behavior of a suspension in a reactor with a lower section which tapers to a point, for example a cylindrical container with a conical lower section which is agitated by gas injection by means of nozzles arranged at a point source or in a line at the tip, has been studied many times; see R. H. Kleijntjens et al. in the Canadian J. of Chem. Engineering 72 (1994), 392-404 and Y. T. Shah et al. in Chem. Eng. Comm. 110 (1991), 53-70. In these types of reactors, there is an upwards directed flow in the region of the stream of bubbles and, parallel to this, a downwards directed backflow near the walls. The solids concentration is at its highest in the lower region of the conical part of the container. These above documents mentioned do not suggest using these types of ascending jet reactors to minimize foam production occurring during a materials conversion process or as a crystallization reactor for continuous crystallization. This type of use, again, is not obvious from the details given in Verfahrenstechnische Berechnungsmethoden, part 4 (1988), chapter 6, in particular pages 158-159, 167-169 and 206-209, since in that document (page 208) reference is made to the foam problem and the use of chemical antifoam agents is recommended for controlling foam.
Accordingly, an object of the invention is aimed at performing chemical and physical materials conversion processes with substances dissolved in a liquid phase and/or suspended therein, and with thorough mixing of the contents of the reactor, in such a way that it is possible to perform the process without any significant foam problems.
A further object of the invention is to achieve the above by using simple engineering means and avoiding or minimizing the addition of antifoam agents.
A still further object of the invention is to improve physical materials conversion processes, and especially a crystallization process, wherein a dissolved substance is crystallized out. Chemical materials conversion processes may be any chemical reactions, in particular those which are performed in the presence of a solid suspended in a liquid medium, for example a solid catalyst, or during which a solid is formed, for instance by reaction and subsequent crystallization or by polymerization.